Quartz is a physically and chemically extremely stable crystal and does not change much with time. Hence, it has been widely used as a quartz vibrator for a long time. The quartz vibrator is an electronic device which takes out mechanical resonance of the quartz crystal as stable electric vibration via piezoelectricity and is essential as a reference clock for operation of an electronic circuit. Besides, the quartz vibrator is widely used as standards for frequency of the oscillator, filter and so on, and utilized in not only information processing devices but also almost all, various electronic devices. Such wide use of the quartz vibrator is due to its temperature-dependent frequency change of the quartz vibrator that is much smaller than that of another electronic device.
When using a vibrator or resonator in an electronic device, the frequency temperature characteristic of the vibrator is extremely important and many efforts have been made to improve this characteristic. The vibrator substrate having the most stable frequency temperature characteristics at present is an AT-cut quartz substrate using bulk waves (thickness-shear waves). Generalized frequency temperature characteristics of the AT-cut quartz vibrator are expressed by a cubic function, and a frequency change is small and about 12 ppm over a general ambient temperature range (−20 to +80 degrees).
In addition, the quartz vibrator using surface acoustic waves has entered widespread use, however, it generally has a drawback of inferior frequency temperature characteristic. For example, an ST-cut quartz widely used as a surface acoustic wave substrate has frequency temperature characteristics expressed by a quadratic function and its frequency change is about 10 times larger than that of the AT-cut quartz.
Meanwhile, the inventors have proposed, as a vibrator using elastic waves different from the conventional surface acoustic waves or bulk waves, a high-frequency resonator using Lamb wave type elastic waves (Patent documents 1 and 2).
However, the frequency temperature characteristics disclosed in these documents show the frequency variation (Δf/f) of about 1.7 ppm over the temperature range of −20 to +80 degrees, which is about one eighth of that of the conventional AT-cut quartz vibrator. When the band of one channel is of hundreds GHz or more, the frequency variation becomes several tens kHz and there may be channel-to-channel interference.
[Patent Document 1] Japanese Patent Application Laid-open No. 2003-258596
[Patent Document 1] Japanese Patent Application Laid-open No. 2005-269284